Change the default Job Type in job submission form to ‘volunteer’

 
add_filter( 'submit_job_form_fields', 'submit_job_form_fields_job_type_default' , 11);
  
function submit_job_form_fields_job_type_default( $fields ) 
{
    $fields['job']['job_type']['default'] = 'volunteer'; // Slug of the job type
  
    return $fields;
}